Document that cpanm can install several modules in one call

[bschmalhofer]

I tried installing multiple modules in a single call.

>cpanm DBI Text::Trim Acme::Bleach
DBI is up to date. (1.643)
Text::Trim is up to date. (1.04)
Acme::Bleach is up to date. (1.150)

This behaviour looks sane to me and I think that it is the intended behaviour. However I did not see this possibility mentioned in the documentation. The ability to install several modules in one go is useful when building Docker images.